Since a catalyst for producing stereoregular polymers had been discovered by Ziegler, Natta et al., crystalline .alpha.-olefin polymers have been produced on an industrial scale. Among the conventional processess for the industrial production of the polymers, a slurry polymerization process has the most widely been used, wherein an .alpha.-olefin(s) is(are) polymerized in an inert liquid solvent.
However, the slurry polymerization process has some drawbacks. For instance, the produced polymer is separated from the solvent and then dried. The separated solvent contains low-crystalline polymers dissolved therein, and hence, in order to re-use the solvent, the dissolved polymers must be removed therefrom. That is, after the removal of the low-crystalline polymers, the solvent is purified and then re-used. Thus, this process requires a complicated procedure and a large amount of energy for the removal of low-crystalline polymers. Moreover, when a polymer containing a large amount of low-crystalline polymers is produced by the process, the low-crystalline polymers are dissolved in the solvent in a large amount, which causes an increase in the viscosity of the polymerization system. In such a case, it is difficult to remove the heat of polymerization, and further, due to adhesion of the polymer particles onto the polymerization vessel, the desired polymer is hard to obtain. To avoid such drawbacks in the slurry polymerization, it has been proposed to polymerize .alpha.-olefin(s) in the gaseous phase in the absence of an inert liquid solvent, but such a gas phase polymerization process still has some problems to be solved to produce a block copolymer having improved impact resistance at low temperatures and improved molding and processsing characteristics.
One of the most important problems in the gas phase polymerization process is that the production of a polymer containing a large amount of low-crystalline polymers, such as a block copolymer having excellent impact resistance, the polymer particles usually adhere easily onto the polymerization vessel and hence it is difficult to obtain the desired polymer in a stable manner. To eliminate this effect, it is necessary to lower the adhesion properties of the polymer particles.
As a reaction vessel for the gas phase polymerization of .alpha.-olefin(s), there are used an agitating mixing type vessel, a fluidized bed type vessel, an agitating fluidized bed type vessel, and the like. However, with an increase of adhesion of the polymer particles, an extremely large force for agitation is required to obtain the desired rotation of the agitator, which causes difficulty in the design of the apparatus. In addition, an occasionally difficulty in the homogeneous mixing of the reaction mixture arises and some portions reach too high of a temperature, which results in the production of a partial chunk injuring the agitator, thermometer, etc. within the reaction vessel or resulting in difficulty in removing the polymer through a pipe from the vessel.
Moreover, in polymerization in the fluidized bed, a slugging phenomenon tends to occur, and hence, a large amount of the polymer particles fly into the gas circulation line and adhere onto the line, which results in closing of the line. There is also occasionally produced undesirable partial chunks due to the difficulty in homogeneous mixing. In the production of polymer particles having high adhesion properties, the pipe for transferring the polymer particles is easily closed. Furthermore, undesirable bridging occurs at the lower part of a cyclone or within the hopper, and hence, the polymer cannot be removed in a stable manner.
Accordingly, it is very difficult to produce a polymer containing a large amount of low-crystalline polymer by a gas phase polymerization process, so that the process has an advantage in that there is not liquid solvent dissolving the low-crystalline polymer.
Another important problem in the gas phase polymerization process is that it is necessary to prevent a wide range of particle size distribution. That is, according to the conventional gas phase polymerization process, to homogeneously mix the reaction mixture and to remove easily the heat of polymerization, a part of the gaseous components is usually drawn out from the top of the reaction vessel and cooled to remove partially or wholly the heat of polymerization and then returned to the reaction vessel. In a typical process where gas components are circulated, there is used a fluidized bed type vessel or an agitating fluidized bed type vessel, wherein a part or whole of the gas is circulated, and thereby, the power necessary for agitation can be saved and the mixture can homogeneously be mixed so as to result in easy removal of the heat of polymerization. However, when the polymerization is carried out by using a catalyst system containing a large amount of fine particles within a reaction vessel wherein the gas components are circulated, the fine particles of the catalyst and also the polymer particles fly within the vessel, which causes some troubles in operation. For instance, the fine particles are accompanied with the gas to be exhausted from the reaction vessel into the circulation line and adhere onto the pipes and devices provided in the line, such as the cyclone, filter, heat exchanger, compressor, flowmeter, and the like, and hence, the devices significantly lose their capacity and occasionally become inoperative due to the closing thereof. Moreover, the fine particles fly to the dilute phase of the polymer within the reaction vessel and adhere onto devices provided therein, such as thermometer. In this phase, the polymerization reaction proceeds to produce chunks which cause pipes to close. With an increase in the chunks, a homogeneous fluidized bed is hard to obtain, which results in a difficult removal of the heat of polymerization and yet a further increase in the amount of chunks. When using an agitating reaction vessel, the agitator as well as other devices, such as a thermometer, are injured by the produced chunks.
When there is used a solid catalyst which contains components having a large particle size, the catalyst particles flow insufficiently within the fluidized bed type reaction vessel, which results in incomplete removal of the heat of polymerization and hence the production of undesirable chunks increases.
There has also proposed an improved gas phase polymerization process. In this improved process, the catalyst residue and atactic polypropylene having undesirable physical properties are not substantially removed, and hence, a specific catalyst system must be used which has highly improved stereoregular properties and polymerization activity.
In addition to the problems during the polymerization process as mentioned above, the obtained polymer particles having high adhesion properties or containing a large amount of fine particles have some disadvantages in the molding and processing steps as mentioned below.
Polypropylene is usually processed into shaped products having various shapes suitable for each desired application, wherein the starting polypropylene is made molten by heating and then molded into the desired shape in a film molding machine, an injection molding machine, or the like. The starting polypropylene is usually used in the form of pellets which are prepared by admixing the polymer particles with conventional additives such as neutralizing agents, heat stabilizers, antioxidants, UV-absorbers, light stabilizers, etc. and melting by heating in an extruder and then molding into pellets. This step of molding into pellets requires a large amount of apparatus and also a large power and energy for heating and extruding.
To eliminate these problems, it has also proposed to directly mold the polypropylene particles admixed with additives without preforming into pellets. However, the direct molding of the polymer particles still has some problems and has rarely been employed practically. One of the main problems is that the particles are not suitable for adaptation to the molding machine, that is, the particles are insufficiently separated at the cyclone of the hopper loader provided in the molding machine and fly into atmosphere. Additionally, when polymer particles having high adhesion properties are molded, the particles adhere onto and close various devices provided in the molding machine, such as pipes for transferring, hoppers, cyclones, etc., by bridging. Thus, it has been desired to improve the properties of the polymer particles for the purpose of molding and processing thereof.